Fiber bulkhead couplers are often used when coupling an optical fiber to an optical device. Such bulkhead couplers often include a collimating lens that must be aligned with respect to an axis of beam propagation to or from a fiber. There are many styles of fiber bulkhead design, however they each suffer from drawbacks that make them less than ideal for applications such as fiber coupled lasers. For example, in a pigtail coupler a fiber is coupled into a ferrule and held in place (typically inside of a sealed laser system). Unfortunately the fiber is not removable. Sealing around this design is typically a permanent adhesive solution. Strain relieving the fiber from the mounting system becomes difficult and misalignment is a common problem. Pigtail couplers typically require special handling and assembly and are not “off-the-shelf”. One alternative fiber coupler is known as an adjustable bulkhead. Such connections are particularly desirable where a collimating lens is required between a fiber and some optical component of a laser or high-power optics. This type of device is a sealed, fiber coupled bulkhead connection with the ability to adjust a collimator lens using some fine pitch (0-80) screws. The spring force for the design is provided by the sealing mechanism—an o-ring. Unfortunately, O-rings are not ideal for holding a micron-level tolerance over time, and this mount is known to drift and be highly imprecise. This type of fiber mount needs to continuously be adjusted through its lifetime, making it an impractical solution for an OEM laser. Although the fiber is removable, the very removal and replacement of the fiber will cause the mount to move, misaligning the system.
Another alternative fiber coupler uses a fixed lens. High precision machining is possible, as are ultra precision optics. It is possible to design a system where a counterbore is precisely located in position and depth in relation to the fiber face. The lens is typically held in a machined bore with adhesive, thereby forming a seal to the outside world. Unfortunately, such a design is expensive to implement. Machining tolerances are exceptionally tight and fundamentally limit the precision of the system. Physical optical tolerances must be excellent, especially in regards to centration. Such tolerances are typically only found and understood in a mass manufactured system, where special tooling has been created to guarantee specifications. Off-the-shelf and low-volume solutions currently on the market are limited in their range of optics and coupling capabilities, and are typically low power solutions only. Effectively, this approach only allows for adjustment in the Z-axis. As used herein, the term “Z-Axis” refers to an axis of beam propagation. The terms “X-Axis” & “Y-Axis” refer to axes that are in a plane perpendicular to the axis of beam propagation.
Many prior art fiber coupling solutions are related to telecommunications. However, manufacturers of precisely aligned and affixed solid-state lasers often have a set of requirements specific to their industry. For example tolerances are tighter in the laser and high-power optics industry than in the telecommunications industry. The very best machined tolerances tend to be on the order of 10 microns. Optical alignments in the laser and high-powered optics industry can be on the order of less than 1 μum. It is impossible to achieve these tolerances with mechanical (i.e. bolts/snaps/references) fixturing alone. Optics must be individually aligned and carefully affixed using a low/non-shrinking adhesive. Solder is occasionally used, but tends to creep or move as it solidifies, and continues to move as the residual stresses relieve themselves.
In addition, long term alignment is critical to the lifetime performance of high-powered lasers and optics. As such, methods that might creep, or retain residual stress are undesirable. Plastics are also undesirable for long term stability. Many laser and high-power optics applications require optical alignments that can maintain tolerances of less than 10 microradians over 40,000 hours of on/off operation. In addition, many lasers and high-power optics are not meant to be “re-tweaked” into position once they have left the factory.
Additional concerns that are not normally found in the telecommunications industry include cleanliness, sealing, thermal insensitivity, and volume. Cleanliness is essential to higher power devices. Most plastics and fixturing methods are not suitable, as they tend to outgas (and fog sensitive optics) causing failure. Sealing typically means more than a simple dust seal. Laser and high-power optics manufacturers often attempt to hermetically seal their packages for reliability. When the fiber connector is removed, the internal optics and system must still be protected. Thermal insensitivity refers to the use of materials with similar coefficients of thermal expansion (CTE) is key for reliable operation. Most plastics have CTE's that are 10 times greater than metals most commonly used in lasers and high power-optics. Finally, most segments of the laser and high-power optics industry, such as the OEM solid-state laser industry, are not high-volume businesses. Consequently, solutions that can be manufactured with typical means and tolerances are desirable. Solutions that allow for the unpredictability of custom optics are also desirable. A solution that uses off-the-shelf connectors is desirable.
Most prior art solutions are related to telecommunications applications that involve low optical power. Most do not mention any method of affixing, and this is a key element of any precision alignment. A majority of the prior art fiber connections are not removable and most are not bulkhead connections. In addition, most are not collimating solutions. Furthermore, most are fiber to fiber only, which limits the use of the connector to coupling between two fibers having the same OD and/or same Numerical Aperture. Cleanliness (outgassing) on a majority of the prior art would be prohibitive for higher powers and long term stability is highly questionable on most.
For example, U.S. Pat. No. 5,857,054 discloses an “Apparatus and Method for Removably Positioning Optical Fiber to Circuit Board Component In Enclosure.” This apparatus is used to align fiber to a detector of large surface area. The tolerances described are an order of magnitude greater than is desirable in many laser and high-power optics applications. The fiber is aligned via a molded in or machined boss, which does not provide as precise an alignment as is often desired. Furthermore, the apparatus does not truly seal, does not allow for alignment in X & Y, and is not designed for coupling light from one fiber to another. In addition, the design is complex and predominately made using plastic. Thus, there are many thermal expansion issues.
U.S. Pat. No. 4,741,591 describes an “Optical Fiber Connector.” This design relies on heat to cure. This is likely to damage coatings used in many laser and high-power optics applications. This design is for a fiber to fiber connection with zero gap only. This would not work for coupling fibers with different diameters or numerical apertures (NA) together. This design does not allow for any Z-axis adjustment. The X & Y adjustment in this design relies on the deformation of a metal part. This would likely have a “spring back” force and be hard to calibrate. In addition, this design requires an elaborate fixture and complex/precise machined parts. Furthermore this design does not seal, is not removable, and is not a bulkhead connector. In addition, this design uses solder, which is notorious for creep and strain. The design also calls for a face-touching contact, which is bad for polished optical surfaces.
U.S. Pat. No. 4,925,265 describes an “Apparatus for Directing a Laser Beam into Optical Fibers.” Unfortunately, this design is not applicable for high power. The fiber connector is not truly sealed. When the connector is removed, optical elements inside are exposed to the environment. Furthermore, the design does not facilitate easy alignment.
U.S. Pat. Nos. 4,666,241, 4,741,590, and 4,614,402 describe a fiber optic connector and method for terminating fiber optic transmission members. These connectors and methods are fiber to fiber only with no means of simple disconnection or reconnecting. Although the design is simple, it is a specialized fiber termination and is not a bulkhead connector. Such a design is not likely to work with different diameters or Numerical Apertures.
U.S. Pat. No. 4,606,603 describes an “Underwater Connector Including Integral Bladder and Seal with Set of Constricting Means.” This is a fiber to fiber connection wherein each fiber has the same diameter and NA. Alignment requires special compression in cylindrical member. The connection is complex, which is not appropriate for small volume manufacturing. The connection requires an index-matching fluid, which is incompatible with many high-power solutions. The index-matching fluid may also change over time.
U.S. Pat. No. 4,925,267 describes a “Structure and Fabrication of Components for Connecting Optical Fibers.” In this design a lens is molded into a tube. This does not allow for coating or polishing the lens. The lens is plastic, which is not compatible with many high-power applications. The lens forming process limits surface quality. The design does not allow for beam sampling between fiber faces and it is not clear whether the connection is disonnectable.
U.S. Pat. No. 5,671,311 describes a “Sealed Multiposition Fiber Optic Connector. This is a low-power solution that uses plastic and relies on ultrasonic welding for fixturing. Alignment is based on a machined/molded boss, not “optical level” tolerances. Each fiber is not disconnectable. Furthermore, the solution is too complex for use with a single fiber.
U.S. Pat. No. 5,700,084 discloses an “Optical Source Position Adjustment Device.” This is a method for aligning an optical source using a feedback loop. This method does not include or describe a method for affixing the optical source. This method also does not affix the optical source in permanent relation to other optics (it is in relation to reference optics only). Furthermore, this method does not provide for a bulkhead connector or repeatable insertion.
U.S. Pat. No. 5,737,349 is directed to an “Optical Isolator and Alignment.” The alignment of an output polarizer is not really a “piston mount”, as the metal output tube is referenced directly to the face and the polarizer is attached to that. Since it is not necessary for a polarizer, the mount does not provide for translation of the polarizer along the Z-Axis and only allows the polarizer to rotate about the Z-Axis. The required translational tolerances on such an optic are very coarse.
Published European Patent Application EP 1 189 241 A1 describes an “Underwater Maintenance Repair Device and Method.” This device uses electromagnets to snug up a fiber connector to a mechanical reference. The electromagnets do not align the connector. Furthermore, any alignment that they do provide would fall apart as soon as electrical power is removed. The device is limited to mechanical level tolerances and it is not evident how this device continues to protect the optics when it is unplugged.
Published European Patent Application EP 1 195 630 A2 describes a “Fiber Optic Connector.” Although this device is sealed and removable it does not allow for alignment along the Z-Axis. Furthermore, this device is a fiber to fiber interconnection only, requiring fibers with the same diameter, and same Numerical Aperture. In addition the design does not facilitate polishing the connector ends.
Thus, there is a need in the art, for a method and apparatus for aligning a lens with respect to an axis of beam propagation that overcomes the above disadvantages.